1. Field of the Invention
The present invention relates to an integrated circuit package.
2. Background Information
Integrated circuits are typically assembled into a package that is mounted to a printed circuit board. There are many types of integrated circuit packages including quad flat pack, pin grid array and plastic ball grid array (PBGA) packages. A PBGA package typically contains an integrated circuit which is mounted to a substrate. A plurality of solder balls are attached to a bottom surface of the substrate and reflowed to mount the package to a printed circuit board. The printed circuit board may be a motherboard of a computer system. The substrate may have routing traces and vias which connect the integrated circuit to the solder balls.
The integrated circuit of a PBGA package may be exposed or enclosed by a protective plastic material. The outer plastic material is typically formed with an injection mold process. The integrated circuit generates heat which must be removed from the package. To facilitate the removal of heat generated by the integrated circuit, it may be desirable to couple a thermal element such as a heat sink to the plastic housing, for example at the top surface of the package.
It has been found that the injection mold process may cause the package to warp. This warpage will create a lack of flatness in the top surface of the plastic housing. The lack of flatness may create air voids between the plastic housing and a thermal element that is mounted to the package. Since air has a relatively low coefficient of thermal conductivity, any air voids in the package/thermal element interface will increase the thermal impedance from the integrated circuit to the thermal element. This increase in thermal impedance may raise the junction temperatures of the integrated circuit.
To maintain the junction temperatures below a threshold design value, it is desirable to remove the air voids between the plastic housing and a thermal element that is mounted to the package. One known technique is an integrated circuit package having a non-grease type of thermal interface material that attaches a thermal element to a package housing mounted on a substrate.
FIG. 1 shows an exploded elevation view of what is known in the art. Integrated circuit package 10 comprises heat sink 12 coupled to integrated circuit 14 that is coupled to substrate 16. Placed between the heat sink 12 and the integrated circuit 14 is thermal interface material 18. Although the use of thermal grease is preferred in reducing thermal resistance between the package and the thermal element, the thermal performance of such grease has been found to degrade primarily through grease pump-out, separation, and dry out. This degradation especially is pronounced after power cycle testing. Alternative materials such as thermally conductive tapes, elastomers, and epoxies have been tried for thermal interface material 18, but have been found to lack the competitive thermal performance of thermal grease. It would be desirable to provide an integrated circuit package that incorporates a thermal grease without creating pump-out, separation, dry out, and other package degrading conditions.